Due to the safety reasons it is necessary to monitor many physical quantities in buildings including vertical displacement of building construction elements, especially the roof construction elements or their parts. It can be realized, among other methods, by determining the difference between current value of distance of these elements from stable construction elements or stable floor and its initial value, which was determined during monitoring initiation, measured by laser rangefinder fastened directly to these elements by means of different kind of suspensions. It is also necessary to monitor a stress in construction elements as a value, which is directly connected with a threat of construction safety. One of the methods of monitoring stress is to monitor element deflection as a value, which is clearly and unambiguously connected with stress (by strain-stress relation) but is much easier to measure.
There are known systems for monitoring changes in the location of building construction elements, in which optical devices are used.
In Polish application description P-381578 a method was described, in which light beam is directed below elements of roof structure. Deflection of at least one construction element equipped with a special diaphragm above the allowable, assumed value results in interruption of light beam and activation of both sound and visual alarms. The device consists of a transmitter and receiver of light beam installed under the roof at opposite ends of a monitored building. Diaphragms, installed on construction elements, are positioned in a straight line above the light beam. Receiver is connected to alarming devices.
In patent description PL 183116 a measuring method of beam torsion and deflection in static and dynamic systems was presented. The method consists in mounting of at least one mirror on a monitored element, on which laser beam is directed, whose reflected light is read on the screen and change of beam stress results in a proportional displacement of reflected laser spot on the screen; in static measurements we get a displacement of light spot and in dynamic measurements—displacement of light line.
Description DE102006045263 reveals a device for measuring of construction deformations, especially construction of the roof, including an optical device emitting signal, e.g. laser, and a device for receiving light signals. The system includes one alarm unit. Measuring devices are fastened on the edges of the building sidewalls and they make measurements of the distance on a straight line between two building walls just below construction elements of the roof.
International application description WO03019113 presents a system, which provides security in buildings, which consists of optical devices emitting light signal and receiving them by means of mirrors. Construction deformation, which causes breaking of a light beam activates an alarm signal.
Description JP 8093230 presents a monitoring method of deformation of roof construction with a system of many laser rangefinders. The system is coupled with a warning system. Laser rangefinders measure distances between construction elements, e.g. elements of the roof.
The main limitation of the above mentioned solutions is the fact, that they are rather detection systems, that means systems, which detect achieving a certain state (value), and not monitoring systems, which can measure continuous changes of a certain quantity, process and archive the collected measuring data. Moreover, in these systems the value, which causes alarm activation, is poorly defined—as an widely understood displacement of elements, which does not have to be an unambiguous measure of stress and thus a threat of construction safety. The additional limitation of these systems can be its insufficient resistance on certain disturbing phenomena, e.g. torsion of elements, to which a mirrors or diaphragms are attached.